From Lambda Dependencies to Lambda Bindings

I’ve had this on the shelf for quite a while, but Kent Boogaart’s article on POCOs vs. DependencyObjects finally got me to cleaning things up a little. Kent is coming up with a similar approach that looks very promising, so you should keep your eyes on his blog, too.

Lambda Bindings are built on top of the Lambda Dependencies project I published a while ago. The original Lambda Dependencies allow you to observe object graphs for changes using simple LINQ expressions. Lambda Bindings leverage this pattern by not just publishing a change event but synchronizing target properties or fields automatically.

This provides you with a generic binding framework that can be used wherever you want to synchronize objects. Let’s have a first example:

What’s happening in the snippet above is that I created a binding between a nested property of a referenced object and a local field. As soon as the binding source (the City property of a school’s address) is changed, the local cityName field is being updated as well.

However, the Lambda Dependencies not only cover the source properties but the whole object graph. Accordingly, exchanging the whole School (or the Student instance) also triggers an update. In the snippet below, the cityName variable is being updated twice:

[Test]
publicvoid Updating_Intermediary_Object_Should_Update_Target(Student student)
{
string cityName = "";
//synchronize the cityName field with the City property of the school's address
var binding = LambdaBinding.BindOneWay(
() => student.School.Address.City,
() => cityName);
//change bound City property -> triggers update of the local variable
student.School.Address.City = "Paris";
Assert.AreEqual("Paris", cityName);
//create a completely different school instance
School englishSchool = new School();
englishSchool.Address = new Address {City = "London"};
//assign the new school to the student
student.School = englishSchool;
//setting the School property also triggered the binding
Assert.AreEqual("London", cityName);
}

Value Conversion

You can do simple value conversion by just submitting a converter to the binding expression. This allows you to intercept the binding pipeline or bind objects of different types together. If you’re coming from WPF, this feels natural anyway, but the solution here does not require you to implement a value converter – a simple Func<TSource, TTarget> is sufficient.

Here’s a simple sample that performs a conversion of a boolean flag to into a corresponding Visibility enum value:

Default Values

In case the object graph is being broken (e.g. because the School was set to null), the target node will be automatically set to its default value (null for an object, 0 for an int etc.). However, you can also specify a default value of your own:

(btw: the above snippet also shows you that you can easily bind to a field rather than a property).

Weak References

The underlying Lambda Dependencies only use weak references so you’re not at risk of creating memory leaks. However, LambdaBinding implements IDisposable, so the proper way to clean things up would be to dispose your binding.

Things to Consider

Remember that that the underlying Lambda Dependencies rely on the INotifyPropertyChanged interface, so don’t expect source binding to fields (or properties that do not fire a PropertyChanged event) to magically update your targets.

Stu,
Hope it’ll help! However, keep in mind, that WinForms controls do *not* implement INotifyPropertyChanged. Accordingly, you can easily update your UI based on events in your Presentation Model, but the dependency framework won’t get notified about changes on the UI due to missing events.
Cheers,
Philipp

Phillip,
I just try to make a unit test in VB, and I found out, that two way binding works, if both ends were properties of objects, but fails, if the ‘source’ is a property and the ‘destination’ is a field. By the way, to define two way bindings, it may be better to name the parameters different than ‘source’ and ‘destination’, isn’t it?

whats about binding between two collections? I think about a scenario, where I get back a collection of Students from my DAL and want to bind a collection(of StudentGUI) to that resultlist? StudentGUI can ba an object which is optimized for GUI usage and Student is a pure DAL object or BO object.

First of all: “Merci vielmol” or thank you very much for this great article!
As you said the code is relying on INotifyPropertyChanged. I’m not a big fan of having this interface on domain objects. Are you thinking of having an extension to the dependency builder where also other pub/sub mechanisms could be supported (e.g. Property “City” has its CityChanged event or similar).

I’ll stick with INPC – after all, it’s the common mechanism for property changes in the .NET framework (and accordingly resides in System.ComponentModel). Of course, a City -> CityChanged event strategy would be possible as well, but this were just another convention, with a more complicated API (10 properties -> 10 different events), so you probably wouldn’t gain too much, would you?